Acquisitions (Narrative) (Details) $ / shares in Units, $ in Millions			1 Months Ended		12 Months Ended
	Dec. 27, 2023 USD ($)	Jan. 22, 2021 USD ($)	Dec. 31, 2022 USD ($) right $ / shares	Jan. 31, 2021 USD ($) right $ / shares	Dec. 31, 2023 USD ($) $ / shares	Dec. 31, 2022 USD ($) $ / shares	Dec. 31, 2021 USD ($)
Business Acquisition [Line Items]
Acquired in-process research and development (Note 3)					$ 3,799.8	$ 908.5	$ 970.1
Acquired IPR&D Expense						$ 333.8
Biopharma Global Inc
Business Acquisition [Line Items]
Business acquisition, share price (in dollars per share) | $ / shares					$ 12.50
Cash paid, net of cash acquired	$ 1,035.8
Akouos Acquisition
Business Acquisition [Line Items]
Business acquisition, share price (in dollars per share) | $ / shares			$ 12.50			$ 12.50
Cash paid, net of cash acquired			$ 327.2
Consideration transferred, number of contingent value rights | right			1
Contingent value right, additional price per share (in dollars per share) | $ / shares			$ 3.00			$ 3.00
Contingent value right, additional price per share, aggregate amount			$ 122.0			$ 122.0
Prevail Therapeutics Inc.
Business Acquisition [Line Items]
Business acquisition, share price (in dollars per share) | $ / shares				$ 22.50
Cash paid, net of cash acquired		$ 747.4		$ 747.4
Consideration transferred, number of contingent value rights | right				1
Contingent value right, additional price per share (in dollars per share) | $ / shares				$ 4.00
Contingent value right, additional price per share, aggregate amount				$ 160.0
Contingent value right, monthly reduction | $ / shares				$ 0.083